Frequency control system



Alfnril 14, 1959 I IJ.. E. BRYDl-:N ETAL 2,882,401

FREQUENCY CONTROL. SYSTEM FiledY March 3, 1955 5 Sheets-Sheet 1 AMPLIFIER FILTER.

- April 14,1959 vv.1. E. RYDEN ETAL 2,882,401

- FREQUENCY coNTRoL'sYsTEM 5 Sheets-Sheet 2 Filed March 3, 1955 VEZ April 14, r1959 Filed March 5, 1955 J. E. BRYDEN ETAL FREQUENCY CONTROL SYSTEMy 5 Sheets-Sheet 3 qrromrey FREQUENCY CONTROL SYSTEM Joseph Easto Bryden, Harrow, and Frederick Charles Flint, Wembley, England, Harry David Hyamson, Mentone, Victoria, Australia, and George Frederick Small, Chesham, England, assignors to The General Electric Company Limited, London, England Application March 3, 1955, Serial No. 492,014 1 Claim. (Cl. Z50-36) The present invention relates to electric oscillation generators.

It is well known to use the resonant frequency of a resonator, for example a cavity resonator, as the frequency reference for controlling the operating frequency of an oscillator. For this purpose a portion of the output from the oscillator is usually supplied to the cavity resonator and either the oscillator is frequency modulated at a predetermined frequency or the resonant frequency of the resonator is varied cyclically at a predetermined frequency. In either case the signal picked up by a probe or other coupling device associated with the resonant cavity is found to be amplitude modulated. This signal is passed to a detector, for example a crystal detector, so as to derive the amplitude modulation, and the component thereof having a frequency equal to the predetermined frequency is then utilised to tune the oscillation generator. The phase of the modulation determines whether the frequency of the oscillator, or its mean frequency, is greater or less than the resonant frequency of the cavity resonator, or its mean resonant frequency, as the case may be and the said component has zero amplitude when the oscillator is correctly tuned.

The present invention is more particularly concerned with oscillation generators in which, for the purpose of effecting automatic frequency control, a resonator has its resonant frequency varied cyclically, and it is one object of the present invention to provide an oscillation generator having a novel system of automatic frequency control.

According to the present invention an electric oscillation generator has an automatic frequency control system which utilises a resonator as the frequency reference, While there is means continuously to rotate a member in the electric or magnetic field of the resonator during operation of the system so as to cause the resonant frequency of the resonator to be varied cyclically over a range of values.

The resonator may, for example, be a cavity resonator or an enclosed co-axial line system. A cavity resonator is one adapted to be excited so as to operate effectively as a closed section of waveguide while an enclosed coaxial line system may be considered as a cavity which has a central conductor and which is arranged to be excited so as to operate as a length of co-axial line that is usually short-circuited at one end and open-circuited at the other.

According to a feature of the present invention, an electric oscillation generator comprises electrical means which is adapted to supply an electric oscillation and which may be tuned, a resonator which is either of the cavity type or an enclosed co-axial line system and to which is arranged to be applied a portion of the electric. oscillation from said electrical means, means to rotate a member in the said resonator at a relatively slow speed so as to cause the instantaneous resonant frequency of the-resonator to be varied, and means to tune the said electrical meansindependence upon the component of lCC the amplitude modulation on the oscillatory electric or magnetic tield in the resonator that has a frequency equal to the frequency at which the resonant frequency of the resonator is varied so as to cause the frequency of the said oscillation applied to the resonator to become substantially equal to the mean resonant frequency thereof.

Preferably an electric motor is arranged to rotate this member at a predetermined speed. The means to tune the electrical means may comprise a two-phase electric motor, the arrangement being such that during operation one phase winding is supplied continuously with alternating current while the other phase winding issupplied with a signal derived from the resonator. The phase of this signal depends upon whether the frequency of the oscillation applied to the resonator is greater or less than the mean resonant frequency thereof and has zero amplitude when they are equal. The two-phase motor is therefore normally stationary but is caused to operate when it is necessary to retune the electrical means. The electrical means may be formed by an oscillator the operating frequency of which is determined by a co-axial line which has a shorting bridge, the position of this bridge being varied by operation of the two-phase motor.

This co-axial line may constitute the main tuning of the oscillator or it may only provide ne tuning.

The rotatable member may be a disc, ring or loop and, if it is of metal, it may vary the resonant frequency of the resonator during operation by disturbing the magnetic field therein. Alternatively it may be of a suitable dielectric material in which case it causes the electric iield to be disturbed.

One arrangement of an oscillation generator for use at frequencies of the order of 2,000 megacycles per second will now be described by way of example with reference to the four figures 0f the accompanying drawings in which Figure 1 shows diagrammatically the electric circuit of the oscillator and the construction thereof,

Figure 2 shows diagrammatically the automaticfrequency control system, and

Figures 3 and 4 show the construction of the resonator which provides the reference frequency of the frequency control system. Figure 3 is a side elevation of the complete resonator partly in section while Figure 4 is a crosssection at the line IV-IV in Figure 3. An electric motor associated with the resonator can also be seen in Figure 3 and a crystal detector which is connected to the resonator is also shown in Figure 4.

The generator comprises an oscillator which is adapted to operate at frequencies of the order of 2,000 megacycles per second. Referring now to Figure l, the oscillator utilises a triode thermionic valve 1 having planar electrodes while its frequency of operation is determined by the effective length of co-axial lines 2, 3 and 4. In fact the inner conductor 5 of the co-axial line 2 is connectedv to the cathode of the valve 1 while its outer conductor 6 is effectively connected at the operating frequency of the oscillator to the grid. The co-axial line 3 surrounds this line 2, the outer conductor 6 of the line 2 forming the inner conductor of the line 3 while the outer conductor 7 of this line 3 is effectively connected at the operating frequency of the oscillator to the anode of they valve 1.

Shorting bridges 8 and 9 of the non-contact making.

type are provided between the conductors of the two co-axial lines 2 and 3 so that the effective electrical length of each line measured from the valve 1 may be varied for the purpose of tuning the oscillator. The bridge 8 may be moved by means of a rod 10 which.

passes through a block 15 of lossy dielectric material while the bridge 9 may be moved by means of studs 16 Patented Apr. 14, 1959y ....ofgqhiol1-sfarihasta@ othofdrowoe) o fomaterial which project through the conductor 7. Two probes 11 and 12 project through the outer condafoltof .thosozoiatlioo is .ia taodoiiforooodootors. of .that .loo and, the out-.- Pil? Sig el., from, tho, sooorafoo is oupplioollby, ooo .of the probesull` oye'rua eirible co-axial transmission line 13 The co-axial yline 4 is loosely coupled to4 thef line 3k throughY the probe 12 and a iiexible co-aXial transmission mio-'11. l. Y

.heHco-aXial line 4 comprises an inner conductor 1`7 .i an outer conductor 18 between which there is ar- 11 w ged to be moved alshorting bridge V190i the non1 co actmaking typeqand theposition of this bridge A1,9 is. ranged to b e moved as .hereinafter described for the purpose of varyingitvhe elective length of the line {hand thereby providing lfine tuning ofthe oscillator; 'H1' .biideolof r'ri'sos tWo 10W .impodanoo oootioos 21 22 which'are separated by a section 23r of relatively highery ir'npetjlancev while a ring 24 `of lossyvv dielectric material isprovided behind the section`22, `The bridge supported by vmeans of six buttons 25 of insulating material (only tivo of these buttons can be seen in the and by the ring 24 Aof lossy dielectric material. lyfovernentI of the bridge 19 is controlled through a mem- 2A6`Vwh`ichfis free tomove in a longitudinal slot in the outer conductor 18 of the co-axial line 4.

A third probe,l which is notsh'own in Figure l, projects through the outer conductor 7 of theUco-axial line 3fland is used to supply a portion of the outputof ie lscillatt'zsr ,toI the resonator which constitutes the freq' "ncy reference of the automatic frequency control sys'tern. This probe is in the plane in which lie the-probes II and' 12 and which is perpendicular to the plane' of Figure A1, this third. probe being vat right angles to the probes 11\and 12 Referring now` to Figure 2, inwhich the anode circuit of the oscillator valve is shown byv a ci le `31thiswg`ure shows the third probe 32V in addition to the probes 11 and 12. The portion of the loutputqosciliation picked up by the probeuSZ is passed through an attenuator 33 to the resonator 34. A i Considering now the construction of the resonator 34 and referring to Figures 3 and 4, the resonator 34 cornprises a: hollow metal cylinder 36 whichiis'closed at biothends to forma cavity and a metal rod 37 lying lly' within the cylinder 36. This rod 37 passes through end ,38 of the cavity which is integral with the cylinder 36 and is electively connected to that end of the cavityt the operating frequency ofthe oscillator, the 11nd 37 beingl accurately located by means of three studs 40 of insulating material. `By means of a micrometer h eadml'l` the` amount of this rod 37 projecting into the eh'ty may be varied. The end of the rod 37 that is u/ithin 4the cavity is spaced an appreciable distance from thejendplate 41 which yis secured over the other end of he' cylinder 36. The cylinder 36 andthe rod y,forni .a length of co-axial line which is shortcuitedqatuone end and open-circuited at the other. 1 'resonant frequency of theresonator this line ,has lectrical length equal to one-quarter wavelength. The resonator has an input probe 42 which is connected to the att nuator 33 (Figure 2) rwhile on the opposite side of hfemcylinder36 there is provided a coupling loop 43 to which isv connected a crystal detector 44. I

he resonant frequency of the lresonator 34 lis ap ranged to be varied cyclically by rotating a copper` disc 45 intllooavifyora Spoed .of 25 .f ovolofionopor Secondhronous velectricy motor 46 is provided for this purp o andl a ceramic coupling 47 is provided between the disc 45 and the motor shaft 48 so as to minimize the leakage o f `iield outside` the cavity. The construction e riiotor'46 isdescrib'ed more fully in thel specifica- ,"of co" application No.A

.into .tho .Spaoogootwooo by the detector 44 is passed .throoehao amelisi?. 15d a filter 41 having a stopv frequency of 100 cycles pei: second to one phase winding of a two-phase electric motor 52. The other phase winding of this motor 52 is connected across the 50 cycle per second supply 53 to which is also connected the motor 46. The motor 52 is arrangedI to control the tuning stub 54, which is formed by the coi-axialvline4 (Figure 1), through a rack and pinion mechanism (not shown). l i A n u Rotation of the disc 45 causes the resonant frequency ofthe resonator 34 to be variedwovc'r a range of vfeque'nciesgvfor example, 4 megacycles per second ofieitler side of its mean frequency, fifty times a second while the angular positiony ofthe disc 45 is unambiguously determined at any instant Aby the phase`of the alternating current supply 53. It will be realised that, when the frequency of theoscillation fed to the `resonator Smis equal to the mean resonant frequency thereof,` there is substantially no componentiin the signal passed by" the" filter 5'1 to the motor 52 having a frequencyof V5'() 'cycles'r'y per second; There is, however, such a componentwljrn' the mean resonant frequency of the resonator 34 dile from the frequency of the applied" oscillation and the"y phasing of this component is arranged to be electiica'l'j degrees leading or lagging on the alternating current supplied to the other winding of the motor 52 dependingupon whether the frequency of the oscillation applied to" the resonator 34 is greater or less than the mean reso nant frequency. In order to obtain this relative phasing it may be necessary to phase-'shift one of the signals` supplied to the motor 52, for eitat'nple,` a phaseshiff network may be provided between the filter 51 and therr motor 52; Alternatively the motor 46 may be mounted-l so that the stator thereof can bev turned relative totl resonator 34 for the purpose of obtaining this phase' relationship. A

Thus, when the frequency of the oscillation applied toi the resonator 34 has a frequency that is equal to' the mean resonant frequency thereof, the two-phase motoi 52 is not operated. Whem however, the frequenc'yofj the applied oscillation has some other value within the pull-in range of the frequency control system, the motor' 52 is caused to operate and to vary the tuning of the oscillator in the correct direction to bring the frequency of the applied oscillation closer to the mean resonant frequency. The traverse speed of the bridge 19 is ap`` proximately 0.22 centimetro per second which cortei' sponds to a rate of' tuning of approximately 11.1 me`gacycles per second. t l Since it is only practicable tojelect fine tuning of the oscillator in this manner over a limited range of fr quencies; mechanical stops may be provided to limit the travel of the shorting bridge 19 while a slip clutch may then be provided between the motor 52 and that bridge. It will be appreciated that on first switching on'uthe generator, the operating frequency of the oscillator niay fall outsidev the range within which the automatic fr4- quency control described above has effect; To over? come this difficulty, there may be provided means to supply alternating current to the two-phase moto iti dependence upon there being no fundamental or second harmonic modulation components'in the signal supplied by the crystal detector 44l so as slowly to tune the oscil lator and bring it within the pull-in range of the frequencyy control system. Y It will be appreciated thataalthoughnthe embodinje described-above makes `use of a-resonator of the eiicl colaxial' line type, a` cavity resonator could alternatvily' be used.

We claim: A An electric oscillation generator comprising electrical means to supply an electricvoscillation, tl''elcct l f resonant, aeeaepata sapete-fs portion of the electric oscillation from said electrical means to the resonator, a circular disc, means to mount said disc to lie wholly within the resonator, a synchronous electric motor arranged to rotate the said disc about a diameter thereof and thereby to cause the resonant frequency of the resonator to vary about a mean value, a two-phase electric motor having first and second phase windings, a mechanical coupling between the two-phase motor and the said tuning member, a detector connected in an electric path between the said resonator and the rst phase winding of the two-phase motor, and means to supply alternating electric current to both the said synchronous motor and the second phase winding of the two-phase motor, whereby when an oscillation supplied by the said electrical means has a frequency equal to that of the mean resonant frequency of the resonator, there is no electric signal supplied to the rst phase winding of the motor and the motor does not therefore drive the tuning member, while if these frequencies are unequal an electric signal which is derived Iby the detector is supplied to the rst phase winding of the motor and the motor does drive the tuning member so as to bring the frequency of the oscillation supplied by the electrical means towards the mean resonant frequency of the resonator.

References Cited in the le of this patent UNITED STATES PATENTS 2,379,689 Crosby July 3, 1945 2,404,568 Dow July 23, 1946 2,521,700 Dodington et al Sept. 12, 1950 2,564,005 Halpern et al Aug. 14, 1951 2,611,092 Smullin Sept. 16, 1952 2,630,488 Clogston Mar. 3, 1953 

